(1) Field of the Invention
The present invention relates to the fabrication of integrated circuits, and more particularly to a method for forming patterned silicon nitride (Si.sub.3 N.sub.4) layers with improved critical dimension (CD) control while minimizing the Si.sub.3 N.sub.4 footing at the edge of the Si.sub.3 N.sub.4 pattern. The method uses an improved plasma etch process that utilizes chlorine as one of the etchant gases.
(2) Description of the Prior Art
Current trends in ultra large scale integration (ULSI) requires sub-micrometer feature sizes to increase the circuit density on the chip. These sub-micron feature sizes require a reliable anisotropic plasma etching of semiconductor materials such as Si.sub.3 N.sub.4, polysilicon, refractory metal silicides, and other semiconductor materials with high aspect ratio and good critical dimension control. One particular area of processing where this is important is the etching of Si.sub.3 N.sub.4 layers on a silicon substrate as an oxidation barrier mask used for selectively growing a thermal oxide by the LOCal Oxidation of Silicon (LOCOS) method commonly practiced in the industry.
However, as shown in FIGS. 1 and 2, problems can arise when open areas are etched in the Si.sub.3 N.sub.4 using conventional photolithographic techniques and conventional anisotropic plasma etching, such as when using a sulfur hexafluoride (SF.sub.6) etchant gas. These problems are best understood by first referring to substrate structure in FIG. 1. The silicon substrate 10 typically has a pad oxide 12 formed on the surface to reduce stress, and then a Si.sub.3 N.sub.4 layer 14 is deposited. A blanket photoresist layer 16 is spin coated and is patterned to leave portions over device areas on the substrate, while exposing the Si.sub.3 N.sub.4 14 in the areas where field oxide (FOX) is desired. Typically the photoresist layer 16 is patterned to have a critical dimension (CD), as shown in FIG. 1. However, residual photoresist A at the bottom edges of the open areas can result in etching problems. And as shown in FIG. 2, during plasma etching of the Si.sub.3 N.sub.4, the photoresist pattern is also laterally recessed a distance X, resulting in an undesirable CD bias in the Si.sub.3 N.sub.4 layer 14 and resulting in a Si.sub.3 N.sub.4 foot or footing A'. Residual photoresist A depicted in FIG. 1 can also exacerbate the problem by resulting in residual Si.sub.3 N.sub.4 A".
Several methods for etching Si.sub.3 N.sub.4 layers with improved profiles have been reported in the literature. In U.S. Pat. No. 5,431,772 to Babie et al. a two-step process is described for etching Si.sub.3 N.sub.4. A first etch is carried out using a gas containing fluorine radicals to remove surface oxide on the Si.sub.3 N.sub.4 layer, and a second etch using a bromine-containing gas is used to selectively etch the exposed Si.sub.3 N.sub.4 to the pad oxide. Kadamura in U.S. Pat. Nos. 5,312,518 and 5,314,576 uses a sulfur fluoride gas and a nitrogen source to form a sulfur nitride based compound that prevents sidewall etching during the Si.sub.3 N.sub.4 etch. Tamaki et al. in U.S. Pat. No. 5,318,668 teach a method of etching a Si.sub.3 N.sub.4 layer using an etchant gas of ClF.sub.3 and HBr which concurrently forms a SiBr.sub.4 to protect the sidewalls from etching while free fluorine is generated for etching the Si.sub.3 N.sub.4. Bialock et al. in U.S. Pat. No. 5,286,344 provide an etch process for selectively etching openings with vertical sidewalls in SiO.sub.2 to a Si.sub.3 N.sub.4 etch-stop layer. In U.S. Pat. No. 4,283,249 to Ephrath, a method is described for selectively etching SiO.sub.2, Si.sub.3 N.sub.4, and silicon oxynitride to a silicon substrate by introducing hydrogen to the CF.sub.4 etchant gas. U.S. Pat. No. 4,803,181 to Buchmann et al. teaches a method for using silylated sidewalls on a patterned photoresist layer and removing the non-silylated photoresist to make an extremely narrow etch mask for patterning underlying semiconductor films. Keller et al. in U.S. Pat. No. 5,387,312 utilize an etch chemistry comprising NF.sub.3 and N.sub.2 that results in a highly selective etch (4-5 times faster) of Si.sub.3 N.sub.4 to pad oxide.
And U.S. Pat. No. 5,338,395 to Keller et al. utilizes a gas chemistry of NF.sub.3 and Cl.sub.2 to provide a predominantly physical sputtering of the Si.sub.3 N.sub.4 layer, and then uses a second etching step using NF.sub.3, HBr chemistry for selectively etching the remaining Si.sub.3 N.sub.4 to pad oxide.
Al though there are a number of methods for etching Si.sub.3 N.sub.4 layers, there is still a strong need to provide an improved method for closely spaced Si.sub.3 N.sub.4 patterns while minimizing the CD bias and providing improved Si.sub.3 o.sub.4 profiles.